The present invention relates to integrated circuit, and more particularly, to integrated circuits for controlling audio signals.
Some electronic appliances include sophisticated audio circuitry and speakers to generate music, voice, and other refined audio signals. However, for many other appliances, generation of such refined audio signals is not necessary. For example, printers, copiers, microwave heaters, and washing machines typically require little more than simple audio circuitry to generate rudimentary sounds such as beeps to alert its operators of certain conditions such as termination of operation or abnormal operations.
FIG. 1A is a simplified schematic illustration of an appliance 10 (a printer, for example) configured to generate rudimentary sounds. In the Figures, relative sizes of various components, structures, or portions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures and configurations of the subject of the Figures.
The appliance 10 includes various electro-mechanical components for performing its function (for printing in the present example). The electro-mechanical components are illustrated as a box 12 and can include, for the present example, rollers, heaters, laser, and sensors. Typically, the electro-mechanical components 12 are connected to and are controller by an integrated circuit chip (IC) 20. The IC 20 is often implemented using an Application-Specific Integrated Circuit (ASIC) chip that includes, within the single chip, a processor and memory. The ASIC 20 is a digital circuit chip programmed to control the operations of the appliance 10 including generations of sounds from a speaker 14.
The speaker 14 is an analog device and it requires an analog audio frequency input signal for operation. Accordingly, an amplifier subsystem 30 is used to drive the speaker 14. The amplifier subsystem 30, is, in turn, controlled by inputs from the ASIC 20. This configuration is illustrated in more detail in FIG. 1B. FIG. 1B illustrates portions of the appliance 10 of FIG. 1A in greater detail. In the illustrated example, the ASIC 20 includes an audio square-wave signal generator 22 providing audio square-wave signals and a volume control circuit 24 providing volume control signals. The audio square-wave signals are sent to the amplifier subsystem 30 via a first pin 21 of the ASIC 20. The volume control signals, each a binary digit, are sent to the amplifier subsystem 30 via a second pin 23 and a third pin 25. The ASIC has additional pins 27 connecting to other parts of the appliance 10 including, for example, the electro-mechanical components 12. For clarity, only relevant parts of the ASIC 20 are illustrated or discussed herein.
The first pin 21 (carrying the audio frequency square-wave signals) is connected to a resistor-capacitor (RC) filter 32 portion of the amplifier subsystem 30. At the RC filter 32, the audio square-wave signals are filtered for reduction of harmonic frequency components. The filtered audio signal is then forwarded to a programmable gain amplifier 34 portion of the amplifier subsystem 30. The programmable gain amplifier 34 has two major parts—a fixed gain amplifier 36 and a gain control circuit 38 connected to the fixed gain amplifier 36.
The second pin 23 and the third pin 25 are connected to a programmable gain circuit of the programmable gain amplifier 34 which determines the level of gain, if any, the fixed gain amplifier 36 applies to the RC filtered audio signal. Finally, the amplified signal is sent to the speaker 14. Here, two binary signals of the volume control signals allow for four volume controls—one zero (off) volume, and three levels of on volume.
To provide additional levels of volume control, additional signal lines (thus pins of the ASIC) are needed. However, in some applications, additional pins may not be available. In fact, it would be desirable to reduce the number of pins required to drive an audio circuit.
Accordingly, there remains a need for a method and apparatus for controlling volume of an audio output while using reduced number of pins for volume control signals.